Hereinafter, a known transmission power control circuit using SIR (signal-Interference Power Ratio) in W-CDMA will be described with reference to drawings.
Conventionally, in transmission power control (hereinafter refereed to as TPC), SIR of a received signal is measured in one of two stations and the SIR is compared with a predetermined SIR value. When the SIR of the received signal is smaller than the predetermined SIR value, a request for increasing a power is output to the other station. On the other hand, when the SIR of the received signal is larger than the predetermined SIR value, a request for decreasing a power is output to the other station.
FIG. 1 shows a known transmission power control circuit which uses SIR of a received signal. The known art shown in FIG. 1 is characterized in that a path selection circuit 3 is not provided and that an SIR addition circuit 12 adds all SIRs (x) input from an SIR estimation circuit 6, compared to an embedment of the present invention (a transmission power control circuit shown in FIG. 2) which will be described later. Now, the known art will be described, concentrating on the difference between the known art and the embodiment of the present invention. Other features will be described in the embodiment of the present invention.
The SIR addition circuit 12 adds all SIRs (x) of all paths after despread. Therefore, for example, when Path (1) and Path (2) are actually one Path and when SIRs (x) include SIR (1)=0.5, SIR (2)=0.3, SIR (3)=0.1, and SIR (4)=0.1, the sum of these SIRs is 1.0.
However, in this case, the SIR of one Path is added twice (Path (1) and Path (2)). Therefore, the true sum of the SIRs must be 0.8.
Herein, when a target SIR value is 0.9, the true SIR is smaller than the target value, and thus a request for increasing a power must be output to the other station. However, the measured SIR is 1.0, which is larger than the target value, and thus a request for decreasing a power is output. As a result, the quality of signal is more and more deteriorated disadvantageously.
An object of the present invention is to provide a transmission power control circuit in which the following problem can be solved. That is, when a path is erroneously detected as a plurality of paths, the SIR value of a received signal becomes larger than a target value. Accordingly, a request for decreasing a power is transmitted to the other station although the line quality is not actually improved. As a result, the line quality is further deteriorated.